A character

The regular expression must match this letter.

A will match the letter A

.

This matches any one character.

"Hack." matches Hack1, Hacki, but not Hack12 or Hackil; only one additional character matches

[]

This matches any one of the characters enclosed in the brackets. The enclosed characters may be a set or a range.

coo[kl] matches cook or cool; [0-9] matches any single digit

[^]

This matches any one of the characters except those that are enclosed in square brackets. The enclosed characters may be a set or a range.

9[^01] matches 92 and 93, but not 91 and 90; A[^0-9] matches an A followed by anything except a digit